The present invention relates to a device for preventing irregular supplying of printing sheets for a printing machine to which printing sheets are supplied one at a time, in which the undesired supply of two or more printing sheets at one time is detected to suspend the sheet supply operation.
In a conventional device of this type, irregular sheet supply is detected by utilizing the fact that, where printing sheets are allowed to pass between a light emitting unit and a light receiving unit, the quantity of light which passes through two or more printing sheets stuck together is smaller than the quantity of light which passes through a single printing sheet. However, when the material or thickness of the printing sheets is changed, or when (as in multi-color printing) sheets printed once are fed to the printing machine again the quantity of light passing through the printing sheet changes. Accordingly, whenever the printing mode is changed, it is necessary to adjust the sensitivity of the light receiving unit. This adjustment is rather troublesome. As the adjustment is carried out using only the intuition of the operator, it is difficult to adjust the sensitivity accurately which may lead to erroneous operations.
An automatic printing machine is known in which a series of steps of loading a original plate on the cylinder, inking, supplying printing sheets, printing and discharging the plate are automatically carried out with a control device such as a microcomputer. If the irregular sheet supply preventing device as described above is used with the automatic printing machine, whenever the original plate is changed (and hence whenever the printing picture is changed) it is necessary to both adjust the sensitivity of the light receiving unit in order to find the most suitable positions of the light emitting unit and the light receiving unit, and to adjust the preventing device itself. These adjustments have prevented the provision of a fully automatic printing machine.